Susceptibility of the mealworm (Tenebrio molitor (L.)) to photodynamic injury by methylene blue

Abstract

Photodynamic injury to protoplasm by photoirradiation in the presence of photically excitable chemicals is perceived as a potentially important phenomenon for entomology and for the ecological–evolutionary implications in host plant – herbivore interrelations.Almost full-grown, larvae of the yellow mealworm, Tenebrio molitor (L.) gave promise as suitable laboratory subjects for studies on photodynamic action. Total mortality resulted from appropriate photoirradiation of methylene blue (MB) injected larvae. A completely fatal dosage of dye consisted of 0.5 μl of 2.5% aqueous MB, injected into larvae weighing 90 to 150 mg, thereby yielding an MB-in-blood concentration of 5 parts/1000. Completely fatal photoirradiation consisted of 90-min exposure to a partially heat-filtered radiation from a 250-W clear reflector infrared lamp with filament 80 cm from the target insect. The resultant mean energy flux was 4.2 μW/cm2 for all points across the action spectrum for MB (500 to 700 nm). This radiant flux was equivalent to about 1/20 of the value for the corresponding spectral band received from the noonday sun in midsummer at latitude 49° N. In future, a tuned source of photoirradiation would be desirable to avoid the complicating heat effect of conventional sources of the unwanted secondary spectrum from a monochromator.